Important early studies on mammalian artificial insemination (AI) were carried out in
equids, and at the end of the 19th century, the first AI programs were set up in horses. At
that time, the most systematic research on equine AI was performed in Russia. After
World War I, AI research shifted to cattle and sheep. This time saw major advances such
as the development of artificial vaginas and phantoms for semen collection. Semen
dilution counteracted the detrimental effect of seminal plasma, allowed semen storage,
and increased the volume of an ejaculate for insemination of more mares. In the late
1930s, techniques for cooled semen AI as used today were in principle available. After
World War II, the number of mares inseminated decreased, but with a new role of the
horse as a partner in equestrian sports, new interest in equine AI was raised. In contrast
to the situation in cattle, frozen semen has not replaced cooled semen AI in the horse.
Recent advances in insemination of horses are the sexing of sperm, low-dose deep
intrauterine insemination, and intracytoplasmic sperm injection